They're back ....

By Janet Hefler - June 8, 2006

The caterpillars have returned with a vengeance this summer, with masses of the feasting insects leaving acres of denuded trees in their slow-moving path. Lace-like canopies of ravaged leaves have replaced the comforting shade of the old oak tree.

"Up-Island, some parts are devastated, completely defoliated," said Pete Costas, controller at Vineyard Gardens. "It is literally like walking through a nest of cobwebs, plus you also have one-inch worms crawling all over you."

The infestation includes not one but several types of caterpillars, with the winter moth, forest tent, and fall cankerworm most prevalent on the Island right now, according to gardening experts.

"If it's any consolation, this probably will be over in the next 10 days or sooner. It should start to subside, said Tim Boland, director at Polly Hill Arboretum.

Like many areas on the Island, the arboretum also is experiencing an infestation, Mr. Boland said. "You go out walking, and you come back looking like a Shell No-Pest Strip, with caterpillars clinging to you," he described.

Despite what feels like a Twilight Zone experience, Mr. Boland said, "We have been trying to make sure people don't become over concerned. Historically, these caterpillars and infestations have been cyclic in nature."

Over time, usually natural predators evolve and increase or caterpillar populations become vulnerable to fungi or bacteriums that eventually start to kill them. Gypsy moths, for example, succumb to a natural fungus that occurs after a spell of damp and rainy weather.

Unfortunately, the winter moth, an invasive species native to Europe, has no natural predators in North America. A program to breed a parasitic fly that will help kill winter moth caterpillars is under development in Massachusetts, but it may take several years before it makes a dent in the numbers.

In the meantime, homeowners want solutions. To spray or not to spray, that is the question.

"Ideally, I wouldn't wait too much longer," said Chuck Wylie, owner of Vineyard Gardens. The caterpillars will be eating for the next two to three weeks, which is the time to target them with spray, he advised.

Mr. Wylie uses Conserve SC, made out of naturally-occurring bacteria, because it is non-toxic to mammals and birds. "In well over 90 percent of the places we've treated, we've sprayed once and had good luck." However, it also depends on timing and the number of caterpillars, he said.

Many people have called Mr. Boland for his advice. "At the arboretum, we have been using Conserve SC on certain trees, especially the irreplaceable ones. What we're doing is protecting trees we can't replace, but not necessarily spraying our oak trees because we have so many of them and it doesn't seem worth it."

Mr. Costas said Vineyard Gardens recommends a similar strategy to help customers keep costs down. "People on a budget may want to have their prize lilacs sprayed and the trees they walk under to get into their house," he explained.

Homeowners should be cautious about trying to spray their own trees or hiring someone for what seems like a bargain price, Mr. Costas warned. "We've heard of people that go up in trees and spray pesticides with a backpack sprayer. We recommend you ask if they have a pesticide license and get specifics about what kind of spray they are using," he said.

People who live in areas with native pitch pines where rare moths species lives, especially near the State Forest, should be cautious about spraying at all, Mr. Boland said. "Even though these are non-toxic pesticides in terms of residual and environment, as a contact kill spray, they will kill native moths that may be rare."

In addition to getting rid of the pests, homeowners are concerned about tree damage. Mr. Boland assures people that the caterpillars likely will not kill a tree with one or two seasons of defoliations. Middle-aged or younger trees will rebound, he said, but will need watering during dry summer months to help them overcome the effects of defoliation.

With a variety of caterpillars comes a variety of appetites. The winter moth caterpillars have a fondness for the foliage of deciduous trees, particularly oaks, and just about everything else, whether roses, perennials,or annuals. Appearing as green inchworms when newly hatched, they balloon from tree to tree or dangle from branches on invisible silken threads.

Forest tent caterpillars feed on native black cherry and crabapple trees, and members of the rose family. Their name is misleading, because they do not make tents. Bluish-black in color, they are fuzzy and have a distinct white keyhole marking along their backs.

The cankerworm caterpillars also hang down on threads in their inchworm stage, and look similar to the forest tent caterpillars when grown, except their back stripes are solid. They also enjoy the foliage of deciduous trees.

Mr. Wylie said the infestations of winter moth and forest tent caterpillars are worse up-Island, while he has seen Gypsy Moth caterpillars only in Vineyard Haven.

A helpful web site for identifying and learning about the caterpillars is available on a UMass Extension web site, www.umassgreeninfo.org. Click on fact sheets, and then "insects and mites," where the caterpillar types are listed alphabetically.